The present invention relates to a method and apparatus for wave soldering printed circuit boards wherein a solder coating is applied only where needed.
A usual wave soldering apparatus includes a pair of endless chain conveyors driven to advance a printed circuit board at a constant speed from the entrance to exit ends of the apparatus. With the printed circuit board held by gripping fingers, the board is first carried to a fluxer where a foam or spray of flux is applied to the underside of the board. The printed circuit board is then carried over preheaters where the temperature of each board is elevated to approximately 110xc2x0 C. to 130xc2x0 C. so as to evaporate excess flux solvent, activate the flux and minimize thermal shock to the printed circuit board. After the printed circuit board is brought to such a preheat temperature, the board is passed over a solder reservoir to receive solder. The board is finally transported to a cool down zone where the solder is cooled to solidify.
Typically, pin grid alley modules and dual in-line packages are loaded onto one side of a printed circuit board, with their terminals or leads projecting downwardly through apertures in the printed circuit board. To increase packaging density, a number of surface mounted devices and connectors are loaded onto the other side of the printed circuit board. Problems have arisen from wave soldering such a printed circuit board. Too much heat occurs on the underside of the board, when contacted with a solder wave, and tends to damage such surface mounted devices and connectors.
Attempts have been made to locally apply solder to preselected conductor areas on a printed circuit board, but not to those areas where surface mounted devices and connectors are mounted. In one known process, flux is locally spayed onto preselected conductor areas on a printed circuit board. The fluxed board is then preheated by a stream of warm gas to evaporate flux solvent. Thereafter, the board is positioned over a plurality of solder wave nozzles arranged within a solder reservoir. At this time, the solder wave nozzles are brought into alignment with a plurality of sets of preselected areas on the printed circuit board. A pump is arranged within the solder reservoir to force heated molten solder to flow upwardly through the solder wave nozzles so as to form solder waves. The preselected areas on the printed circuit board are contacted with the respective solder waves to make soldered joints.
This known process has proven to be effective for eliminating heat damage to sensitive electronic components such as surface mounted devices, but is unable to provide a high degree of productivity since all the steps involved in this process must be manually carried out on a batch basis.
Accordingly, it is an object of the present invention to provide a method and apparatus for locally applying solder to preselected conductor areas on a printed circuit board on a fully automated basis.
According to one aspect of the present invention, there is provided a method for locally applying solder to preselected conductive areas on each printed circuit board. A first printed circuit board is loaded onto a pair of first rails located above a flux station. The flux station includes a plurality of flux nozzles arranged in a pattern substantially identical to that of a corresponding plurality of sets of preselected conductive areas on the first printed circuit board. The flux nozzles are activated to locally apply flux to the sets of preselected conductive areas on the first printed circuit board. Then, the first printed circuit board is transported from the flux station to a preheater station, and at the same time, a second printed circuit board is loaded onto the first rails. The preheater station is located downstream of the flux station and below the first rails. Thereafter, the preheater station is activated to preheat the first printed circuit board, and the flux nozzles are activated to locally apply flux to a corresponding plurality of sets of preselected conductive areas on the second printed circuit board. The first printed circuit board is removed from the preheater station and then, loaded onto a pair of second rails below which a wave solder station is located, the second printed circuit board is transported from the flux station to the preheater station, and at the same time, a third printed circuit board is loaded onto the first rails above the flux station. The wave solder station is located downstream of the preheater station and includes a plurality of solder nozzles arranged in a pattern substantially identical to that of the sets of preselected conductive areas on the printed circuit boards. The second rails are located downstream of the first rails. The solder nozzles are activated to locally apply solder to the sets of preselected conductive areas on the first printed circuit board, the preheater section is activated to preheat the second printed circuit board, and the flux nozzles are activated to locally apply flux to a plurality of sets of preselected conductive areas on the third printed circuit board.
Preferably, the flux station is moved toward the first rails when the flux is applied. Also, the second rails are moved toward the wave solder station when the solder is applied.
According to another aspect of the present invention, there is provided an apparatus for locally applying solder to a plurality of sets of preselected conductive areas on a printed circuit board. The apparatus comprises a flux station including a plurality of flux nozzles arranged in a pattern substantially identical to that of the respective sets of preselected conductive areas on each of the printed circuit boards for locally applying flux thereto, a preheater station located downstream of the flux station and arranged to preheat the printed circuit boards, and a wave solder station located downstream of the preheater station and including a plurality of solder nozzles arranged to locally apply solder to the respective sets of preselected conductive areas on each of the printed circuit boards. The flux station, the preheater station and the wave solder station are arranged in line in the direction of conveyance of the printed circuit boards. The apparatus further comprises a conveyor system for simultaneously transporting the printed circuit boards from one station to another along a substantially horizontal conveyor path. The conveyor system includes a fixed frame extending in the direction of conveyance of the printed circuit boards, an elongate arm carriage carried by and reciprocably moved along the fixed frame, a plurality of pairs of pivot arms pivotably connected to the arm carriage and movable between an inoperative position where the pivot arms are disengaged from the printed circuit boards and an operative position where the pivot arms are engaged with the printed circuit boards, a pair of first rails connected to the fixed frame and located above the flux station and the preheater station, and a pair of second rails connected to the fixed frame and located above the wave solder station.
Preferably, the preheater station includes a plurality of gas outlets. The flux nozzles, the gas outlets and the solder nozzles are equally spaced from each other by a first distance, and the pivot arms are equally spaced from each other by a second distance substantially equal to the first distance.
The frame preferably includes a horizontally extending connecting member located below the arm carriage and adapted to pivotably connect the pivot arms. The frame further includes an actuator mounted to the arm carriage and operatively connected to one pair of the pivot arms.
The conveyor system further includes an upstream conveyor belt located upstream of the flux station and selectively driven to transport the printed circuit boards, and a downstream conveyor belt located downstream of the wave solder station and selectively driven to transport the printed circuit boards.
Preferably, the first rails are fixedly connected to the frame, and the second rails are movably connected to the frame for movement away and toward the wave solder station.
Preferably, the flux station includes a lifting device adapted to move the flux station toward and away from the first rails.